Lineage specific cells and progenitor cells

ABSTRACT

A method for generating a culture that is purified or enriched in respect of cells of a selected lineage is described in which selectable marker, which is differentially expressed in cells of the selected lineage compared with its expression in other cells, is introduced into a multipotential cell and the multipotential cell is cultured to induce differentiation of the multipotential cell into a cell of the selected lineage, or into a mixture of cells including cells of the selected lineage, or is cultured to induce preferential survival of cells of the selected lineage. Those cells that express the selectable marker are then selected for. Progenitors of selected lineage are also described as is the use of the method in assay techniques.

This is a continuation of application Serial No. PCT/GB99/01136, filedApr. 14, 1999.

This invention relates to lineage specific cells and progenitor cells,methods of obtaining them and their uses. Pluripotent embryonic stem(ES) cells can be induced to differentiate in vitro into a mixture ofcell types, comprising extraembryonic yolk sac and derivatives of allthree embryonic germ layers. However, the disorganised and heterogeneousnature of the differentiation impedes manipulation and analysis ofindividual lineages. In particular, the invention provides alineage-specific genetic selection technique to establish purifiedpopulations of neural precursors from differentiating ES cells.

Embryonic stem (ES) cells are non-transformed cell lines deriveddirectly from the pluripotent founder tissue in the mouse or humanembryo, the epiblast (Evans and Kaufman, 1981; Martin, 1981; Brook andGardner, 1997; Thomson et al, 1998; Shamblott et al, 1998). ES cells canbe propagated and extensively genetically manipulated whilst retainingthe capacity for multilineage differentiation, both in vivo and in vitro(Robertson, 1987). The differentiation of ES cells in culture closelyreflects differentiative events in the embryo. In principal, therefore,ES cells provide in vitro access to the instructive and selectiveprocesses by which cellular diversification is generated in themammalian embryo (Smith, 1992).

Multilineage differentiation of ES cells can be initiated by simpleaggregation (Martin and Evans, 1975; Doetschman et al., 1985). Theaggregates form structures known as embryoid bodies, differentiation ofwhich mirrors aspects of peri- and early post-implantation mouseembryogenesis (Martin et al., 1977; Doetschman et al., 1985). A diversearray of cell types are subsequently found in outgrowths from embryoidbodies (Weiss and Orkin, 1996). Although representation of particularlineages can be diminished or enhanced by treatment with dimethylsulphoxide or retinoic acid (RA), the differentiated products alwaysconsist of a mixture of cell types. This intrinsic disorganisation andcomplexity have limited the exploitation of in vitro ES celldifferentiation for assignment of gene functions in developmentalpathways.

Several reports have documented the presence of neuronal cells and glia(Bain, 1995; Fraichard et al., 1995; Strubing et al., 1995; Okabe etal., 1996) in embryoid body outgrowths. This could be exploited todetect, characterise and manipulate factors that regulate neurogenesisand neuronal and glial differentiation. ES cells could also be used as asource of normal or genetically manipulated neural cells for biochemicaland functional studies or for transplantation. The problem is that theseobjectives, however, are severely compromised by the abundance ofnon-neural cells in the cultures and because it is not currentlypossible to establish or maintain a culture predominantly containingneural cell progenitors.

The art thus fails to provide a reliable method of obtaining asubstantially pure population of a cell of any selected lineage, andprogenitor cells of a selected lineage in particular. The art also failsto provide assays for developmental and other effects of factors onprogenitor cells or differentiated cells of a selected lineage.

The present invention aims to provide progenitor cells and/ordifferentiated cells of a selected lineage, assays for the effect offactors on such cells and uses of these cells, such as intransplantation. The present invention further aims to provide a geneticselection technique to eliminate non-neural cells from cultures and toenable purification from embryoid bodies of differentiation competentneural precursors.

The present invention provides a method of obtaining a culture of cellsof a selected lineage having a combination of two steps. One step is toselect for cells that express a gene characteristic of cells of thatlineage. The other is to so culture cells that they tend todifferentiate into or proliferate as cells of that lineage. The effectis to yield a more highly purified culture of the selected-lineage cellsthan is otherwise possible.

By way of example, one step of the combination is to select for cellsthat express a gene known to be expressed uniquely in haematopoieticprogenitor cells, and the other is to culture cells in medium containinga nutrient known to promote differentiation of cells into haematopoieticprogenitors.

Accordingly, the invention provides a method for generating a culturethat is purified or enriched in respect of cells of a selected lineage,comprising:—

-   -   (i) introducing into a multipotential cell a selectable marker        that is differentially expressed in cells of the selected        lineage compared with its expression in other cells;    -   (ii) culturing the multipotential cell to induce (a)        differentiation of the multipotential cell into a cell of the        selected lineage or into a mixture of cells that is or includes        cells of the selected lineage or (b) preferential survival of        cells of the selected lineage; and    -   (iii) selecting for those cells that express the selectable        marker,

The method is suitable for obtaining cells, optionally progenitors, ofthe selected lineage at high purity. Whilst following the art method forinducing differentiation of ES cells into a culture that includes neuralcells can provide a maximum of about 50% neural progenitors, using thetechnique of the invention a purity in excess of 70% can be obtained,and in a specific embodiment described below the purity is substantially100%.

Step (ii) results in a pure population of cells or a mixed culture atleast slightly purified or enriched in respect of desired cells and cansuitably be carried out by culturing the multipotential cell in thepresence of a factor that induces differentiation of the cell into aprogenitor cell of the selected lineage. By way of example, a mitogenspecific for neural progenitors is fibroblast growth factor. Referenceherein to the term “factor” is not intended to be limited to protein orpolypeptide factors but is intended to encompass any biologically activemolecule or potentially biologically active molecule.

The multipotential cell may be selected from embryonic stem (ES) cells,embryonic germ (EG) cells, embryonal carcinoma (EC) cells, a primaryculture of foetal cells, a primary culture of post-natal cells and aprimary culture of adult cells. The method may further comprisegenetically modifying cells to delete, mutate, substitute or add genesin order to assay gene function in progenitor cells of the selectedlineage or adapt a cell phenotype to render it more suitable fortransplantation. The cells may thus be obtained by introducing aselectable marker into a multipotential cell line or a primary culturecontaining the cells of interest and then selecting out the cells ofinterest. The selectable marker is optionally introduced by transfectionor viral infection via a transgenic animal from which the primarycultures are then established, suitably using the methods described inWO-A-94/24274.

The invention thus advantageously enables a highly enriched populationof cells and in particular all lineages of progenitors of a chosenlineage to be obtained. In an example below the population obtained issubstantially 100% pure making it possible to isolate a single cell ofknown progenitor lineage. The invention is of application to alllineages of cells, particularly progenitor cells. A selectable markerexpressed in cells that express a Sox gene leads to neural progenitorcells; the CD34, CD44 and SCL genes are suitable for obtaininghaematopoietic progenitors, and the Nkx 2.5 or GATA-4 gene for cardiacprogenitors. For generating myogenic progenitors, Myo0 or myE5 aresuitable. Using retinoic acid induces differentiation of ES cells intoneural cells, DMSO induces differentiation into haematopoietic cells andabsence of retinoic acid induces a population enriched in cardiacprogenitors.

The method optionally further comprises:—

-   -   (iv) introducing into the multipotential cell a second        selectable marker that is differentially expressed in progenitor        cells or other cells of a selected sub-lineage compared with its        expression in other cells, wherein cells of the selected        sub-lineage are formed by differentiation of cells of the        selected progenitor lineage; and    -   (v) when a culture of progenitor cells of the selected lineage        has been obtained, allowing or inducing differentiation of the        cells and selecting for cells that express the second selectable        marker.

This aspect of the invention further enhances the purity of the obtainedculture of cells, and is of advantage in cases that cells of theselected lineage differentially express a gene but only at a levelslightly different from non-desired cells.

In a preferred embodiment of the invention, described in more detailbelow, the selectable marker is a gene that codes for antibioticresistance and selecting for those cells that express the selectablemarker comprises introducing antibiotic into the culture. In use,application of the antibiotic selectively kills or ablates cells that donot express the marker, leaving behind a population of cells purified orenriched in respect of those expressing the antibiotic resistance, i.e.viable cells of selected lineage. At least two ways of introducing theselectable marker are known and suitable for the invention. Theselectable marker may be introduced into the multipotential cell bytargeted integration or gene trapping into a gene that is differentiallyexpressed in progenitor cells of the selected lineage. Expression of theselectable marker is thereby operatively linked to a gene differentiallyexpressed in a desired pattern. The selectable marker may also beintroduced into the multipotential cell via random integration as atransgene wherein it is expressed under control of the regulatoryelements of a gene that is differentially expressed in progenitor cellsof the selected lineage.

The selectable marker may more generally be a marker that when expressedresults in preferential survival of cells expressing the marker, withantibiotic resistance being one such example. In this instance themarker is expressed in cells of the selected lineage. The selectablemarker may also be a marker whose expression results in preferentialkilling of cells expressing the marker. In this instance the marker isexpressed in cells other than those of the selected lineage.

In a specific embodiment of the invention described in an example below,the multipotential cell is an ES, EC or EG cell and the method comprisesinducing differentiation of the multipotential cell—one way is to forman embryoid body from the cell—and dissociating the cells. Trypsin isused in one example. It is an advantage that individual cells arethereby obtained. These are all exposed to the culture medium and notbeing attached to neighbouring cells are free of cell-to-cell influencesthat might affect the pattern of growth and/or differentiation of thecells, and hinder formation of progenitor cells according to theinvention.

A culture that is purified or enriched in respect of ventral progenitorcells is obtainable according to the invention, wherein the selectablemarker is differentially expressed in neural progenitor cells and thesecond selectable marker is differentially expressed in ventralprogenitor cells. The second selectable marker for this is suitablydifferentially expressed in cells that express Pax 6.

A culture that is purified on enriched in respect of dorsal progenitorcells is obtainable according to the invention, wherein the selectablemarker is differentially expressed in the neural progenitor cells andthe second selectable marker is differentially expressed in dorsalprogenitor cells. The second selectable marker for this is suitablydifferentially expressed in cells that express Pax 3.

The invention also provides a cell, preferably a progenitor, of aselected lineage, obtainable according to the method of the invention.Hitherto, preparations of progenitors were too impure for certainty asto whether any chosen cell was a progenitor cell. With culture accordingto the invention that can give rise to substantially 100% purepreparations of progenitors, isolation of a single progenitor isachieved.

The invention further provides a composition comprising a plurality ofcells, wherein a majority of the cells are progenitor cells of aselected lineage. Preferably, at least 60% of the cells are progenitorcells of the selected lineage. More preferably, at least 60% of thecells are neural progenitor cells. In addition, the invention providesan isolated neural progenitor cell.

A significant application of the invention is in the field of assayingfactors for the effects they may have on a selected progenitor.Accordingly, the invention still further provides an assay of the effectof a factor on a culture of progenitor cells of a selected lineage,comprising:—

-   -   (i) introducing into a multipotential cell a selectable marker        that is differentially expressed in cells of the selected        lineage compared with its expression in other cells;    -   (ii) culturing the multipotential cell to induce differentiation        of the multipotential cell into a cell or mixture of cells that        includes cells of the selected lineage or to induce preferential        survival of cells of the selected lineage;    -   (iii) selecting for those cells that express the selectable        marker; and    -   (iv) culturing the thereby obtained cells of selected lineage in        the presence of the factor.

The assay method is preferably for assay of the effect of a factor on aculture of progenitor cells of selected lineage, wherein the selectablemarker is differentially expressed in those progenitor cells. Referenceto a “factor” in the assay, is intended to be a reference to any actualor potential biologically active molecule that may be introduced intoculture of cells of the selected lineage, and is thus not intended to belimited to protein and polypeptide factors. The term is also intended toencompass a gene introduced or modified in a multipotential cell fromwhich cells of the selected lineage are obtained. The assay can thusconveniently be used to assay for the effect of a particular gene, aswell as for the effect of polypeptide and/or protein factors or smallmolecules of interest.

The method can be used to assay whether the factor has a proliferative,maturation, toxic or protective effect on progenitor cells of theselected lineage, or whether a factor has a proliferative, maturation,cytotoxic or glial protective effect on neural progenitor cells or onother differentiated cells obtained following withdrawal of selectionand differentiation of the progenitors.

It is an advantage of the assay method that it enables a population ofterminally differentiated cells of selected lineage to be obtained andused for such assays as gene and drug discovery screens.

Yet further provided by the invention is a neural progenitor cell, or aculture comprising a majority of neural progenitor cells, fortransplantation. In an example below, a neural progenitor cell of theinvention has been successfully isolated and transplanted into the brainof a rat. The neural cell may optionally be a neuronal cell or a glialcell.

This transplantation can also be carried out using neuronal cellsobtained from neural progenitors of the invention. A suitable method ofgenerating purified neurons comprises obtaining a culture purified inrespect of neural progenitors, using the method of the invention whereinthe selectable marker is differentially expressed in cells that expressa Sox gene, and culturing the progenitors obtained in the presence ofmedium suitable for differentiation of the progenitors into neurons. Asuitable method of generating a purified glial cells comprises obtaininga culture purified in respect of neural progenitors, using the method ofthe invention wherein the selectable marker is differentially expressedin cells that express a Sox gene, and culturing the progenitors obtainedin the presences of medium suitable for differentiation of theprogenitors into glial cells.

Yet further the invention provides a neural progenitor cell fortransplantation to treat neurodegenerative disease or neuronal/braininjury, a neural progenitor cells for transplantation, obtainable from acell selected from an ES cell, an EC cell, an EG cell a primary cultureof foetal cells and a primary culture of post-natal cells, fortransplantation to treat neurodegenerative diseases or neuronal/braininjuries, a method of treatment of neurodegenerative disease orneuronal/brain injury comprising transplantation of a neural progenitorcell, and a method of preparing a neural progenitor cell or adifferentiated progeny thereof for storage, comprising obtaining thecell in a method according to the invention and freezing the cell in thepresence of a cryoprotectant, such as 10% dimethyl sulfoxide.

In an embodiment of the invention, a selection/reporter gene, βgeo, wasintegrated by homologous recombination into the sox2 gene, which isexpressed uniformly in precursor cells in the neural plate and neuraltube. Application of G418 to differentiating cultures of sox2-targetedcells resulted in the efficient isolation of viable βgeo-positive cells.These cells expressed markers of neuroepithelial precursor cells. Theydifferentiated efficiently into networks of neuron-like cells thatexpressed a variety of neuronal markers. Thus, an in vitro system forgenetic and molecular dissection of mammalian neural differentiation isprovided by the invention, and also a route for the production of purepopulations of normal or genetically modified neural precursors andneurons for functional studies including transplantation. Furthermore,the lineage selection approach of the invention is applicable to theisolation of other precursor populations from differentiating ES cellcultures.

The invention still further provides a method of amplifying a purifiedpopulation of progenitor cells of a selected lineage, comprising

-   -   maintaining the cells in culture in the presence of        -   a mitogen; and        -   a growth factor.

The progenitor cells preferably comprise a gene coding for a selectablemarker, which gene is differentially expressed in the progenitor cellscompared with its expression in other cells, and the method furthercomprises selecting for cells that express the selectable marker. It isan option to maintain the culture over a plurality of generations andperiodically select for cells that express the selectable marker.

Another option is to maintain the culture over a plurality ofgenerations and continuously select for cells that express theselectable marker. Where the selectable marker is antibiotic resistancethe method may thus comprise continuous culture in the presence ofantibiotic.

The invention is now described with reference to the accompanyingdrawings in which:—

FIG. 1 shows morphology and characterization of neurons and glia in day4 cultures;

FIG. 2 shows phase contrast picture of day 0 cells (2 a—4 hoursfollowing plating), and Sox1 antibody staining (2 b)—scale bar is 50microns;

FIG. 3 shows a schematic diagram of lineage selection according to theinvention;

FIG. 4 shows culture enrichment for Sox2-positive cells by G418selection;

FIG. 5 shows expression of region-specific neural precursor markers;

FIG. 6 shows Sox2 positive cells proliferate in vitro in response toFGF-2;

FIG. 7 shows ES cell-derived neurons differentiated followingSox2-selection;

FIG. 8 shows results of assay of effect of sonic hedgehog on neuralprogenitors; and

FIG. 9 shows an ES cell-derived neuron after transplantation in utero.

In more detail, FIG. 1 shows morphology and characterization of neuronsand glia in day 4 cultures. E14TG2a ES cells were induced todifferentiate in aggregates for 8 days, plated on poly-D-lysine/lamininsubstrate in DMEM/F12 supplemented with N2. Cells were photographedalive with phase-contrast optics after 4 days in culture. The majorityof the cells are neuronal-like, their neuritic processes connected intoa cellular network. (a), (b) and (c) show cultures that were stainedwith anti-NFL, anti-MAP2, and anti-GFAP, respectively.

In FIG. 4, ES cells with a targeted insertion of βgeo into the Sox2 genewere induced to differentiate by culture as aggregates (“embryoidbodies”). After 4 days they were exposed to 10⁻⁶ M retinoic acid for afurther 4 days. The aggregates were then dissociated and cells plated onpoly-D-lysine and laminin-coated dishes in serum-free medium with N2supplement. Cultures were unselected or exposed to G418 either for thefinal 48 hours of aggregate culture (b, f, h, i) or for 24 hours afterplating (d):

-   -   a. Sox2-linked βgalactosidase expression in unselected culture 3        hours after plating.    -   b. Sox2-linked β-galactosidase expression in G418-selected        culture 3 hours after plating.    -   c. Sox2-linked βgalactosidase expression in unselected culture        24 hours after plating.    -   d. Sox2-linked β-galactosidase expression in G418-selected        culture 24 hours after plating.    -   e. Immunostaining with anti-Sox2 of unselected culture 3 hours        after plating.    -   f. Immunostaining with anti-Sox2 of G418-selected culture 3        hours after plating.    -   g. DAPI counterstaining of panel e.    -   h. DAPI counterstaining of panel f.    -   i. Double-labelling with anti-nestin (green) and anti-Sox2        (red/orange) of G418-selected culture 3 hours after plating.

The scale bar indicates 100 μm for (a, b, c, d), 66.7 μm for (e, f, g,h) and 50 μm for (i).

For FIG. 5, Sox2 expressing cells were selected with G418 for 2 days inEB cultures, cells were dissociated and allowed to attach to culturedishes for 3 hours before fixation. The expression for Pax3 (a), delta-1(b), Mash-1 (c) and Math-4a (d) were detected by in situ hybridisationwith specific anti-sense riboprobes; the presence of Pax6-expressingcells were detected by staining the culture with an anti-Pax6 antibody(e), culture e was double labelled with DAPI to localize nuclei of allcells (f). Scale bar, 100 μm.

For FIG. 6, following 8 days induction of neural differentiation, cellswere dissociated by trypsinization, plated at low density, and culturedin DMEM/F12 supplemented with N2 medium in the presence of 10 ng/ml ofbFGF and 200 μg/ml of G418. (a) overnight culture; (b), 4 day culture.Cell numbers increased around 10 fold over the culture period. Cellsexpanded in the presence of bFGF maintained Sox2 expression as shown viaX-gal staining. Scale bar: 50 μm.

For FIG. 7, Sox2-expressing cells selected by 48 hours exposure to G418during embryoid body culture were plated and grown in the absence ofG418 in N2 medium for 48 hours (a and b) or 96 hours (c–e), or for 72hours followed by a further 72 hours in Neurobasal medium plus B27supplement and 2% horse serum (f–i) (Scale bar, 100 μm):

-   -   a, b. Double immunolabelling showing down-regulation of        Sox-2 (a) in newly differentiating cells expressing the neuronal        marker β-tubulin 3 (b).    -   c. Immunostaining for neuronal marker, β3-tubulin with propidium        iodide counterstaining showing neuronal differentiation of >90%        of cells.    -   d, e. Double immunolabelling for neuronal markers synapsin-I (d)        and MAP2/Tau (e).    -   f. Immunostaining for GABA    -   g. Phase contrast image of field in f.    -   h. Immunostaining for glutamate.    -   i. Phase contrast image of field in h.

In FIG. 8, CCE-sox2 ES embryoid bodies undergoing g418 selection wereexposed to the indicated concentrations of recombinant sonic hedgehogprotein. Cells were fixed and immunostained for the motorneuron markerislet ½ 48 hours after dissociation.

In FIG. 9, sox-2 expressing cells selected by exposure to g418 duringembryoid body culture were dissociated, labelled with PKH26-GL (Sigma)and injected into the forebrain vesicles of E16 rat foetuses in utero.Pregnancy was then allowed to go to term. Pups were sacrificed on P2.Vibratome sections were prepared and examined by fluorescencemicroscopy. The figure shows a representative PKH26-GL labelled cellwithin the cortex exhibiting typical immature neuronal morphology. Thisstudy demonstrates that Sox2 selected cells can integrate anddifferentiate in the brain.

Materials and Methods

ES Cell Culture

ES cell lines used in this study were: E14TG2a (Hooper et al., 1987),CGR8 (Mountford et al., 1994) and CCE-Sox2, a derivative of CCE (Bradleyet al., 1984) in which one copy of the Sox2 gene has been disrupted byhomologous recombination. All ES cell lines were maintained ingelatin-coated tissue culture plastic in Glasgow modified Eagle's medium(GMEM) supplemented with 10⁻⁴ 2-mercaptoethanol, 10% fetal calf serum(FCS) and 100 U/ml LIF (Smith, 1991).

Induction of Differentiation.

The basic protocol is based on that described (Bain, 1995). ES cellswere lightly trypsinized into small clumps and allowed to aggregate insuspension culture in the absence of LIF. After 4 days culture, alltrans-retinoic acid (RA) was added at a concentration of 10⁻⁶M. After afurther 4 days aggregates were dissociated by incubation with trypsinand trituration. Cells were seeded at 3×10⁵ cells/well into 4-welldishes (Nunc) coated with poly-D-lysine and laminin. Culture medium wasserum-free DMEM/F12 (50/50) supplemented with N2 and, where specified,B27 (Gibco-BRL) plus 2% horse serum.

Substrate Preparation.

Tissue culture plates were precoated with poly-D-lysine (PDL, 30–70 kDa,Sigma) for 20 min at a concentration of 10 μg/ml in PBS. Excess PDL waswithdrawn and the plates were rinsed with PBS three times before coatingwith laminin (Sigma) at a concentration of 2–10 ?g/ml in PBS overnightat 4° C.

Immunocytochemistry

Staining for Sox1 and Sox2 was carried out on cultures fixed in MEMFA(4% formaldehyde, 100 mM MOPS pH 7.4, 20 mM EGTA, 1 mM MgSO₄) for 1hour. To stain cells with antibodies against GABA and glutamate, cellswere fixed with 1% glutaraldehyde in PBS (Turner, Neurochemistry). Fordetection of other intracellular antigens, cultures were fixed in 4%paraformaldehyde in PBS for 15 min. Fixed cells were rinsed with PBS,incubated with blocking buffer (PBS, 1 mg/ml BSA, 0.1% Triton X-100, and1% goat serum) for 30 min followed by incubation with primary antibodiesin blocking buffer overnight at 4° C. Cells were then rinsed with PBS,and incubated with a species specific secondary antibody in blockingbuffer for 1 hr. Cultures were washed with three changes of PBS, mountedwith Vectashield mounting media (Vector) and examined under afluorescent microscope.

Double labelling experiments were performed by simultaneously incubatingcells in appropriate combinations of primary antibodies, followed byincubation with non-cross-reactive secondary antibodies. In someexperiments, cultures were counter stained with propidium iodide or DAPIat concentrations of 1 ng/ml and 5 μg/ml respectively.

The following dilutions were used for primary and secondary antibodies:rabbit anti-Sox1 and anti-Sox2 (1:500), rabbit anti-GABA (1:2000,Sigma), rabbit anti-glutamate (1:4000, Sigma), mouse anti-β-tubulin 3(1:200, Sigma); mouse anti-NF68 (1:400, Sigma); rabbit anti-MAPs, whichreacts with MAP2 and Tau (1:400, Sigma); mouse anti-GFAP (1:400, Sigma),mouse anti-synapsin-I (1:50, Chemicon), mouse anti-nestin (1:50, DSHB),mouse anti-Pax6 (1:10, ---). Cy3-conjugated goat anti-mouse IgG (1/50,Jackson ImmunoResearch), FITC-conjugated goat anti-mouse IgG (1:100,Sigma), FITC-conjugated goat anti-rabbit 1 g (1:100, Sigma),TRITC-conjugated goat anti-rabbit Ig (1:50, Sigma).

In Situ Hybridization of Cultured Cells.

The protocol is based on that of (Rosen and Beddington, 1993) adaptedfor cultured cells. Briefly, cultures were fixed in 4% paraformaldehydeand permeabilized at −20° C. in 100% methanol. The cells were thenrehydrated through a series of methanol and finally put into PBS with0.1% Tween-20. Prehybridization was performed for 1–4 hr inhybridization buffer (50% ultrapure formamide, 5×SSC, pH 4.5, 50 mg/mlheparin, 100 μg/ml herring sperm DNA and tRNA, 0.1% Tween-20), thedigoxigenin (DIG)-labeled probes were added at 1 μg/ml overnight at 70°C. Washes the following day were three times for 30 min at 65° C. inwash buffer (50% formamide, 2×SSC, 0.1% Tween-20), three 5 min washes inTBST (137 mM NaCl, 25 mM Tris-HCl, pH 7.6, 3 mM KCl, 0.1% Tween-20) and1 hr block in 10% serum/TBST. Cells were then incubated overnight at 4°C. with alkaline phosphatase-coupled anti-DIG antibody (1:2000,Boehringer-Mannheim). The following day cells were washed three times 1hr with TBST and three times for 10 min with alkaline phosphatase buffer(APB, 100 mM NaCl, 100 mM Tris pH 9.5, 50 mM MgCl₂, 0.1% Tween-20). Thealkaline phosphatase staining reaction was allowed to proceed for 3 hrto overnight with 4.5 μl/ml NBT an 3.5 μl/ml BCIP in APB (Boehringer).

DIG-Labeled RNA Probes

The murine cDNAs used as templates for riboprobes were a 519 bp Pax3fragment (provided by Dr Rosa Beddington), a 700 bp Delta-1 clone(provided by Dr Domingos Henrique), a 670 bp Mash-1 fragment and a 1.5kb Math-4A cDNA (both were provided by Dr Francois Guillemot). DNAs werelinearized and RNA synthesis was directed using T7, T3 or SP6 RNApolymerase, including DIG-labeled nucleotide mix as recommended by thesuppliers (Boehringer). Products were analyzed on a 0.8% agarose gel andapproximately 1 μg/ml of the DIG-labeled antisense RNA was used forhybridization of cells.

Detection of β-galactosidase

Cells were fixed in fix buffer (0.2% glutaraldehyde, 0.1 M phosphatebuffer pH 7.2, 2 mM MgCl2, 5 mM EGTA) for 10 min at 4° C. and washedthree times with wash buffer (0.1 M phosphate buffer pH 7.2, 2 mMMgCl₂). The cells were then incubated at 37° C. overnight with 1 mg/ml5-bromo-4-chloro-3-indolyl-β-D-galactoside (X-gal), 4 mM potassiumfericyanide, 4 mM potassium ferrocyanide in wash buffer (Beddington etal., 1989).

Results

Efficient Generation of Neurons and Glia from ES Cells

Induction of neural differentiation was based on published methods(Bain, 1995; Fraichard et al., 1995) in which ES cells are aggregated insuspension to form embryoid bodies, exposed to retinoic acid, and thenallowed to reattach and outgrow on a substrate. Under these conditionsneuronal like cells become evident in the outgrowths after several daysaccompanied by a variety of other cell types. We introduced twovariations into the protocol which enhanced the final representation ofneuronal cells. First, the embryoid bodies were dissociated beforeplating. This results in a homogeneous dispersion and immediatelyterminates inductive and selective effects within the embryoid bodies.Second, cells were plated in a defined neuronal culture medium (DMEM/F12plus N2) on substrates coated with poly-D-lysine and laminin whichsupport attachment and outgrowth of neuronal cells. Each of theseprocedures had an additive effect on the proportion of neural cells inthe cultures. When combined, more than 50% of viable cells 4 days afterplating had extended processes and where immunoreactive for neuronalmarkers neurofilament light and heavy chains, MAP2/tau, or β tubulin III(FIG. 1 and data not shown). A smaller proportion of cells, around 20%,expressed the astrocyte marker GFAP (not shown). Significantly thisobservation is not cell line specific as comparable results wereobtained with three independent ES cell lines and several subclones.

Non-neural cell types remained in these cultures, however, oftenidentifiable as large, flat non-refractile cells. If, at any point,DMEM/F12 plus N2 was supplemented with serum or mitogens (FGF-2 or B27supplement), these non-neural cells expanded rapidly and progressivelybecame the predominant cell type.

Detection of Sox1/Sox2 Expressing Neural Progenitors

The generation of a large proportion of differentiated neurons and gliasuggested that neural precursor cells might be detectable at an earliertime point in the differentiation protocol. FIG. 2 a shows arepresentative culture 4 hours following plating. At this stage, cellsappear to be morphologically undifferentiated. The soma of the majorityof cells are small, elongated or oval shaped; Some cells have shortprocesses similar in length to their cell bodies. These morphologicalfeatures are similar to primary neural precursors reported previously(Kalyani et al., 1997). These cells do not express detectable NF-L orGFAP 3 hours after plating. After overnight culture, less than 1% of thepopulation were positive for either of these two markers.

The cultures were examined for presence of the intermediate filament,nestin (Lendahl et al., 1990), which is expressed in neuroepithelialcells. Almost all cells were nestin-positive, however, presumablybecause the expression of nestin is not strictly restricted toneuroepithelium [Hockfield, 1 985 #868]. We therefore sought a morespecific marker. The SRY-related transcription factor Sox 1 is confinedto the neuroepithelium of the neural plate and dividing neuralprogenitors in the early mouse embryo (Pevny, unpublished data). Therelated Sox2 gene is expressed in an overlapping pattern that alsoencompasses floor plate and early neural crest cells. Immunostaining offreshly plated cells with antibodies raised against Sox1 and Sox2revealed that 40–50% of the cells were positive (FIG. 2 b and FIG. 3 e).These cells likely correspond to neural progenitors.

Selection and Purification of Sox2-Expressing Neural Progenitor Cells.

To isolate the neural progenitor pool we used ES cells in which thebifunctional selection marker/reporter gene βgeo has been integratedinto the Sox2 gene by homologous recombination (ref). When induced todifferentiate as described above, approximately 50% of these cellsstained for β-galactosidase activity (FIG. 2 and Table 1), consistentwith the proportion of cells that express Sox2 protein. We applied G418to the differentiating cultures to eliminate Sox2-negative non-neuralcells (FIG. 3).

G418 (200 μg/ml) was added after retinoic acid induction, either duringembryoid body culture or upon plating. In both conditions appreciablecell killing was evident. Crucially, however, large numbers of cellssurvived that exhibited typical neuroepithelial morphology. Over 90% ofthese cells gave prominent β-galactosidase staining (FIG. 4).Concordance with Sox2 protein expression was confirmed by immunostaining(FIG. 4, Table 1). The related HMG-box factor Sox1, an exclusive markerof pluripotential neural plate stage cells and of CNS-restrictedprecursors in the neural tube, was detectable in the great majority ofcells. Almost all viable cells expressed nestin.

Sox2-Selected Cells Express Markers of Neural Progenitor Specification.

The developmental organisation and subdivision of the central nervoussystem is underpinned by the temporal and spatial patterning of geneexpression (Tanabe and Jessell, 1996). In order to gauge the diversityof neural differentiation that could be achievable from ES cells in theabsence of embryonic axial organisation, we have begun to examineexpression of key determinants, Pax genes and neurogenic bHLHtranscription factors.

The paired box transcription factors Pax3 and Pax6 are found in dividingneural precursors throughout the length of the embryonic neural tube.Pax3 is initially expressed in the neural plate and subsequently becomesconfined to the dorsal half of the neural tube (Liem et al., 1995). Pax6is expressed predominantly in the ventral region of the neural tube(Walther and Gruss, 1991; Tanabe and Jessell, 1996). Widespreadexpression of both pax genes was found in sox-2 selected cells analysedon the day of plating (FIG. 5 and Table 2). This suggests that EScell-derived neural precursors can acquire both dorsal and ventralidentities.

The bHLH genes, mash1 and math4A (neurogenin) show a more restrictedlocalisation to subsets of neural progenitors. Expression of each wasreadily detected in sox2-selected cultures, but in significantly fewercells than the pax gene products (FIG. 5, Table 2). It is likely thatthe expression of mash1 and math4A specifies distinct sub-populations ofprogenitors, as the distribution of these two transcription factors ismutually exclusive in most CNS regions (Gradwohl et al., 1996).

Two early markers of neuronal differentiation were also examined. Thenotch ligand delta1, which is found in committed cells immediately priorto neuronal differentiation (ref), was expressed in only 1–2% of cellsimmediately after plating, indicating that the majority of the cells arenot yet committed to terminal differentiation (see also Discussion).Expression of the LIM homeodomain protein islet-1/2, an early marker ofdifferentiation of motor neurons and ventral interneurons (Ericson etal., 1992), was examined in Sox2-selected cultures 48 hours afterplating. Reproducibly 1–2% of cells were immunoreactive at this stage(not shown).

Sox2-Selected Cells Proliferate in Response to FGF-2.

Several studies have presented evidence that basic fibroblast growthfactor (FGF-2) can support proliferation of primary neural progenitorsand immortalised progenitor cell lines (Palmer et al., 1997). Additionof FGF-2 (10 ng/ml) to Sox-2 selected cultures likewise stimulated cellproliferation. FIG. 6 shows a typical Xgal-stained culture followingaddition of FGF-2. It can be seen that all cells retain relativelyundifferentiated morphology and show strong Xgal staining. Such culturescould be expanded and serially passaged for at least two weeks.

Neuronal Differentiation of Sox2-Selected Cells.

In order to determine whether the Sox2-selected precursor cells retainedthe capacity for neuronal differentiation, G418 was removed from themedium. Within 48 hours the cells began to extend neuritic processes andby 96 hours a network of neuron-like cells had formed (FIG. 7).β-galactosidase activity was lost from the majority of cells (not shown)consistent with the down-regulation of Sox2 in differentiating neurons(ref). Immunostaining confirmed the disappearance of Sox2 protein (FIG.7 b). Pan-neuronal markers neurofilament light and heavy chain (notshown), microtubule associated proteins (MAPs/tau), β-tubulin III (Leeet al., 1990) and synapsin I were detectable from 48 hours onwards,co-incident with down-regulation of Sox2. By 96 hours over 90% of cellshad long dendritic processes and expressed neuronal markers (FIG. 7).Supplementation of the culture medium with B27 and horse serum allowedfurther maturation of the neuronal cells, evidenced both by increasedsprouting of dendrites and by production of excitatory and inhibitoryneurotransmitters GABA and glutamate (FIG. 7).

The mammalian nervous system is derived from neuroepithelial cells ofthe neural tube and its derivative neural crest. During neurogenesis,these neural progenitors proliferate, progressively lose theirmultipotentiality and finally differentiate into various type of postmitotic neurons and glial cells [Anderson, 1993 #673, McKay, 1989#672(Tanabe and Jessell, 1996; McKay, 1997)]. The mechanisms involved in thedetermination and differentiation of neural precursor cells are thesubject of intense investigation. This has been hitherto hindered,however by the relative inaccessibility and tissue complexity of themammalian embryo. An in vitro model system of the present invention inwhich neuroepithelial cells can be derived and undergo proliferation anddifferentiation provides a powerful tool to study intrinsic andextrinsic factors that determine neural specification anddifferentiation.

ES cells have the capacity to develop into any cell type as evidenced bytheir colonisation of all lineages in chimaeras (Beddington andRobertson, 1989). The prospect of using ES cells to dissectdevelopmental pathways in vitro has been frustrated, however, by aninability to control or direct differentiation. In an embodiment of theinvention described above, there is provided a strategy for selectingprecursors of the lineage of interest from developing embryoid bodies.Our results demonstrate that viable neural precursors can be isolated byselection for Sox2 gene activity. These cells can be induced toproliferate or to differentiate into neurons. Therefore the survival anddevelopment of the neural lineage in vitro does not require continuedinteraction with other cell types. Moreover, the finding thatsignificant cellular components can be ablated without causingdisintegration of the embryoid bodies, nor apparently perturbingdevelopment of the surviving cells is both surprising and encouragingfor the application of this approach to other lineages. Indeed it ispossible that such ablation may favour the maintenance and expansion ofspecific stem cell populations by removing sources of differentiationinducing signals, which may be either cells from other lineages or moremature cells of the same lineage (Mountford and Smith, in personalcommunication).

The heterogenous expression of determination genes (FIG. 5) within theSox2 selected cultures appears reflective of the subspecification ofprogenitors within the neural tube. Requirements for induction, growthand differentiation of individual progenitor types could therefore beinvestigated, both by addition of extracellular regulators to theculture system and by genetic manipulation of the ES cells prior todifferentiation. The latter particularly relevant to situations wherehomozygous gene deletion or transgene expression in vivo cause embryoniclethality. In addition, the ability to produce genetically modifiedneurons is likely to find significant applications in neuronal cellbiology and biochemistry.

It has recently been reported that differentiated ES cells injected intothe developing (Brustle et al., 1997) or even adult (Deacon et al.,1998) rodent brain can colonise the host nervous system and give rise tomature neuronal phenotypes. Such transplants also contain non-neuronalcells, however. These foreign cells can give rise to teratomas and otherbenign or malignant growths. They may also interfere with trophicsignalling and guidance cues from host tissue to injected neural cells.Prior isolation of the neural precursors according to the inventioneliminates these problems. Furthermore, following application of lineageselection purified neural cells can be accessed at any stage of in vitromaturation and harvested for transplantation.

The invention opens the way for development of human multipotential stemcells analogous to the mouse ES cells of an example above for clinicaluse in transplantation. Neurodegenerative conditions such as Parkinson'sand Huntington's diseases are potentially treatable by cell replacementstrategies and present compelling cases for development of a renewablestem cell resource for production of transplantable cells (Svendsen andRosser, 1995; Rosenthal, 1998). The lineage selection approach, incombination with appropriate instructive factors, is a valuablecomponent of such a system, by enabling the generation of a defined cellpopulation from a multipotent source.

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TABLE 1 Expression of neuroepithelial markers following G418 selection−G418 +G418 % positive staining Markers (mean +/− SEM) β-galactosidase43.8 ± 9.4   91 ± 2.6 Sox2 48.4 ± 1.1 95.5 ± 1.1 Sox1 46.6 ± 5.6 88.7 ±5.1 Nestin*   90 ± 6.5   94 ± 3.2

G418 was applied to EB cultures from the 6th day of induction at aconcentration of 200 μg/ml. Two days later aggregates were trypsinizedand the cell suspension was plated on poly-D-lysine/laminin coatedsubstrate in DMEM/F12 plus N2. Cultures were fixed 3 hours after platingfollowed by histochemical staining for β-galactosidase with X-gal orimmunocytochemical staining for Sox1, Sox2 and nestin with specificantibodies. Positively stained cells were scored under a 40x objective,seven to ten fields were counted for each sample. The result is given asan average percentage from two independent experiments (*Nestin isexpressed in somitic mesoderm in addition to neuroepithelium).

1. A method for generating a culture that is purified or enriched inneural progenitor cells, comprising: (i) introducing into a pluripotentcell a selectable marker that is differentially expressed in neuralprogenitor cells compared with its expression in other cells, whereinneural progenitor cells constitute a sub-set of the cells obtainablefrom the pluripotent cell, and wherein expression of the selectablemarker is under the control of a Sox gene promoter; (ii) culturing thepluripotent cell in vitro to induce differentiation of the pluripotentcell into a neural progenitor cell or into a mixture of cells includingneural progenitor cells, or to induce preferential survival, in a mixedculture of cells, of neural progenitor cells; and (iii) selecting forneural progenitor cells according to differential expression of theselectable marker introduced in step (i).
 2. The method according toclaim 1 wherein the pluripotent cell is selected from embryonic stem(ES) cells, embryonic germ (EG) cells, embryonic carcinoma (EC) cells, aprimary culture of fetal cells, a primary culture of post-natal cells,and a primary culture of adult cells.
 3. The method according to claim 1comprising genetically modifying pluripotent cells by deleting,mutating, substituting or adding genes in said pluripotent cells inorder (i) to assay gene function in neural progenitor cells, or (ii) torender selected cells more suitable for transplantation, or both.
 4. Themethod according to claim 1 further comprising: (iv) introducing intothe pluripotent cell a second selectable marker that is differentiallyexpressed in cells of a selected sub-lineage compared with itsexpression in other cells, wherein cells of the selected sub-lineage areformed by differentiation of neural progenitor cells; and (v) when aculture of neural progenitor cells has been obtained, allowing orinducing differentiation of the cells and selecting for cells thatexpress the second selectable marker.
 5. The method according to claim 1wherein the selectable marker is introduced into the pluripotent cell bytargeted integration or random gene trap integration so as to beoperatively coupled to a gene that is differentially expressed in neuralprogenitor cells.
 6. The method according to claim 1 wherein theselectable marker is introduced into the pluripotent cell via randomintegration of a transgene in which the selectable marker is operativelycoupled to a gene that is differentially expressed in neural progenitorcells.
 7. The method according to claim 1 wherein the pluripotent cellis an ES, EG or EC cell and the method comprises forming an embryoidbody in step (ii), or otherwise inducing differentiation of the cells.8. The method according to claim 7 wherein the differentiated cells aredissociated so as to form a culture comprising of individual cells. 9.The method according to claim 7 wherein differentiated cells of anembryoid body are dissociated using a protease, such as trypsin.
 10. Themethod according to claim 1 wherein the Sox gene is selected from Sox 1,Sox 2 and Sox
 3. 11. The method according to claim 1 wherein theselectable marker is an antibiotic resistance gene and the methodcomprises culture in the presence of antibiotic.
 12. A method ofpreparing a neural progenitor cell or a differentiated progeny thereoffor storage, comprising obtaining the cell in a method according toclaim 1 and freezing the cell in the presence of a cryoprotectant.
 13. Amethod of generating purified neuron cells, comprising obtaining aculture of neural progenitor cells using the method of claim 1 whereinthe selectable marker is differentially expressed in cells that expressa Sox gene, and culturing the progenitor cells obtained in the presenceof medium suitable for differentiation of the progenitor cells intoneuron cells.